1. Technical Field
The present invention relates to a shift control device for a continuously variable transmission.
2. Related Art
Continuously variable transmissions (CVTs), such as belt type CVTs and toroidal CVTs, have come into widespread used in recent years as automotive automatic transmissions (e.g., see Japanese Unexamined Patent Application Publication No. 2004-125072). Continuously variable transmissions enable non-step gear ratio change, without speed shift shock, and can improve gas mileage. A belt type continuously variable transmission includes a primary pulley provided to an input shaft, a secondary pulley provided to an output shaft, and a drive transmission component such as a belt or chain, running over the pulleys. The gear ratio is steplessly changed by changing the groove width of the pulleys, thereby changing the diameter of the position where the drive transmission component rides.
In such continuously variable transmissions, the gear ratio is controlled based on throttle position (accelerator position) and vehicle speed, engine speed, or some other parameter indicating the running state of the vehicle. Target primary pulley rotation number is set based on these parameters, and the gear ratio is set such that the actual primary pulley rotation number changes to be the same as the target primary pulley rotation number.
However, continuously variable transmissions have a problem that in transience during gear shifting, when the accelerator pedal is depressed and the rotations change, the engine torque is absorbed by the inertia of the pulleys (engine torque is consumed in raising the rotations of the pulleys), and so the amount of torque consumed is torque lost. This means that when the driver steps on the accelerator pedal, the torque is not immediately available, and so the vehicle accelerates with a delayed response. In such situations, the driver may feel that the intended acceleration is not being attained and further step on the accelerator, resulting in excessive acceleration, so the driver then has to back off of the accelerator. Conventional continuously variable transmissions have had this problem that there is a difference between the acceleration which the driver intends and the acceleration actually obtained, which may result in the driver having an uncomfortable sensation.
Also, there has been a problem that kick-down control start determination (transition condition) has conventionally been performed based on the accelerator position and accelerator opening rate, so kick-down control might not be initiated if the driver slowly depresses the accelerator pedal. That is to say, kick-down might not be initiated if the accelerator pedal is depressed slowly, even if the accelerator position and the depression amount are the same.